The present invention is related to digital communication systems and more particularly to systems and methods for controlling output power of subscriber units in a point to multipoint communication system.
A point to multipoint wireless communication system represents a potentially effective solution to the problem of providing broadband network connectivity to a large number of geographically distributed points. Unlike optical fiber, DSL, and cable modems, there is no need to either construct a new wired infrastructure or substantially modify a wired infrastructure that has been constructed for a different purpose.
In order to conserve scarce spectrum, the data communication devices of a point to multipoint wireless communication system may share access to a common frequency. In a typical scenario a first group of one or more frequency channels are allocated to downstream broadcast communication from a central access point to a plurality of subscriber units. A second group of one or more separate frequency channels are allocated to upstream communication from the subscriber units to the central access point. For upstream communication there is a medium access control (MAC) protocol that determines which subscriber unit is permitted to transmit at which time so as not to interfere with transmissions from other subscriber units.
For a given upstream frequency, the time domain is divided into frames that are typically of equal duration. Each frame represents an individually allocable unit in the time domain. One subscriber unit transmits in each frame. Reservations for transmission in a particular frame are made by the central access point and distributed in broadcast downstream transmissions. Such a scheme is referred to as a time domain multiple access scheme (TDMA).
In such a point to multipoint wireless communication system, it is generally preferable to centrally control the transmission power of each subscriber unit. Each subscriber unit should transmit at a power sufficient to ensure accurate reception of its transmission yet not so high so as to overload the front end of the central access point's receiver or cause interference to unintended receivers. Power control involves monitoring subscriber unit transmitted power at the central access point and sending power adjustment information downstream to maintain power at the desired level.
Cable modem systems also require access to a shared medium and subscriber unit power control. It would be desirable to simply adopt a MAC protocol already developed for cable applications to the wireless context. One such protocol that has been developed is referred to as the MCNS protocol. The MCNS protocol is described in the Data-over-Cable Service Interface Specifications, Radio Frequency Interface Specification, SP-RFI-I04-980724, (Cable Television Laboratories, 1997), the contents of which are herein incorporated by reference.
A cable MAC layer like MCNS is already implemented in low cost chip sets. The operational characteristics of MCNS are well known. Furthermore, it is desirable to maintain parts commonality between wireless modems and cable modems to the extent possible.
The MCNS protocol provides for controlling the power of subscriber units. In one implementation, the power control function is combined with monitoring of the round trip propagation delay between the central access point and individual subscriber units. Periodically, the head end sends a ranging request message to a particular subscriber unit. In response to the ranging request message, the subscriber unit sends a ranging response to the central access point. The time of the response indicates the subscriber unit's current understanding of the system clock as modified by the propagation delay. The head end then tells the subscriber unit by how much to adjust its clock phase to align its transmissions to the system MAC layer clock maintained by the head end. The head end also measures the power level of the ranging response message. Also, the head end sends the subscriber unit power adjustment information to help the subscriber unit set its power so that it will be received at a desired level.
This combined ranging and power control operation is, however, relatively infrequent, occurring approximately every two seconds in typical implementations. This MAC layer power control operation cannot easily be made more frequent because of the limited processing power provided by equipment implementing the MCNS protocol. In a wireless system, the frequency of power control operations that may be implemented practically with MCNS is insufficient. Channel response may vary too rapidly for the MCNS power control system to react. If hundreds of milliseconds have passed since the last update to the subscriber unit's power level, new data transmitted by the subscriber unit may be included in a transmission having an either excessive or insufficient power level.
One solution is to measure at the central access point the power of other subscriber unit transmissions such as data transmissions, access request transmissions and/or special power measurement transmissions. Such techniques are not admitted prior art to the present application and are discussed in the co-filed, co-assigned applications entitled POLLING FOR TRANSMISSION POWER CONTROL and OPTIMAL USE OF REQUEST ACCESS TDMA SLOTS FOR AUTOMATIC LEVEL CONTROL.
It is desirable to base power control on power level measurements obtained from disparate types of upstream transmissions that may occur at irregular intervals. For example, consider a voice over IP application where the network is supporting a voice telephone call made from a subscriber unit location. Upstream data transmissions from the participating subscriber unit will be relatively frequent and power measurements based on successive data transmissions will be up to date for the duration of the call. However, when a subscriber unit has been inactive for a long time the most recent power measurement may be based on a very old transmission. What is needed is a power control technique that can optimally exploit power measurements that occur at irregular intervals.